Color correction system and method

ABSTRACT

A system and method for calibrating a digital imaging device for color correction is disclosed. The method comprises obtaining an input color value and a reference color value for each of a plurality of color references, as well as a noise evaluation image having a color noise for evaluating noise reduction, the input color values and reference color values being in a non-linear color space. A plurality of color correction parameters are determined as optimized based on evaluating a fitness function in the non-linear color space. The non-linear color space can be a CIE L*a*b* color space. The fitness function can include a color correction error and a noise amplification metric so as to reduce noise amplification during color correction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to, copendingPCT Patent Application Number PCT/CN2015/079094, which was filed on May15, 2015. The disclosure of the PCT application is herein incorporatedby reference in its entirety and for all purposes.

COPYRIGHT NOTICE

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

FIELD

The disclosed embodiments relate generally to digital image processingand more particularly, but not exclusively, to systems and methods forcolor correction of digital images.

BACKGROUND

Because digital imaging devices acquire colors differently from the waythat human eyes perceive color, images acquired by digital imagingdevices typically benefit from color correction. However, the colorcorrection process may be prone to introducing and/or amplifyingdifferent types of noise. This is the general area that embodiments ofthe invention are intended to address.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary top-level block diagram illustrating anembodiment of a color correction apparatus for color-correcting adigital image.

FIG. 2 is an exemplary diagram illustrating an embodiment of an imagingsystem including the color correction apparatus of FIG. 1, wherein theimaging system is shown imaging a color reference using an image sensor.

FIG. 3 is an exemplary diagram illustrating an alternative embodiment ofthe imaging system of FIG. 2, wherein the color correction apparatus isshown as acquiring various color values for calibration of colorcorrection parameters.

FIG. 4 is exemplary top-level flow chart illustrating an embodiment of amethod for calibrating a digital imaging device.

FIG. 5 is exemplary flow chart illustrating an alternative embodiment ofthe method of FIG. 4, wherein color correction parameters are optimizedfor calibrating a digital imaging device.

FIG. 6 is exemplary flow chart illustrating an alternative embodiment ofthe method of FIG. 5, wherein the method includes a two-stepoptimization method.

FIG. 7 is exemplary flow chart illustrating another alternativeembodiment of the method of FIG. 5, wherein the method includes agenetic process.

FIG. 8 is an exemplary diagram illustrating an embodiment of the methodof FIG. 5, wherein the method includes sampling at different spatialfrequencies to determine a noise amplification metric.

FIG. 9 is an exemplary diagram illustrating an embodiment of the methodof FIG. 8 with spatial downsampling.

FIG. 10 is an exemplary flow chart illustrating an embodiment of themethod of FIG. 5, wherein the method includes sampling at differentspatial frequencies to determine a noise amplification metric.

FIG. 11 is an exemplary diagram illustrating an embodiment of an imagingsystem installed on an unmanned aerial vehicle (UAV).

FIG. 12 is an exemplary diagram illustrating a chrominance diagramshowing a color error of a first experiment testing the efficacy ofoptimizing color correction parameters with noise regulation.

FIG. 13 is an exemplary diagram illustrating noise values of theexperiment of FIG. 9 testing the efficacy of optimizing color correctionparameters with noise regulation.

FIG. 14 is an exemplary diagram illustrating a chrominance diagramshowing a color error of a second experiment testing the efficacy ofoptimizing color correction parameters without noise regulation.

FIG. 15 is an exemplary diagram illustrating noise values of theexperiment of FIG. 11 testing the efficacy of optimizing colorcorrection parameters without noise regulation.

FIG. 16 is an exemplary diagram illustrating a two-step method foroptimizing color correction parameters with noise regulation.

It should be noted that the figures are not drawn to scale and thatelements of similar structures or functions are generally represented bylike reference numerals for illustrative purposes throughout thefigures. It also should be noted that the figures are only intended tofacilitate the description of the preferred embodiments. The figures donot illustrate every aspect of the described embodiments and do notlimit the scope of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is illustrated, by way of example and not by way oflimitation, in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” or “some” embodiment(s) in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone.

Color correction is a process that transforms signals acquired byphotosensors of digital imaging devices into colors that look realistic.Color correction is a transformation defined by a set of colorcorrection parameters. These parameters are typically calibrated foreach individual digital imaging device to find customized parametervalues that accurately reflect the color response characteristics of theindividual device.

The calibration of color correction parameters entails an optimizationprocess in which colors values acquired by the imaging device arecompared with known reference color values. Typically, the goal duringthe optimization process is to minimize a difference between theacquired colors post-color-correction and the known reference values. Adrawback of this approach, however, is that accounting for colorcorrection accuracy alone can often result in parameters thatexcessively amplify noise. Image noise can include color and brightnessvariations in an image. These variations are not features of an originalobject imaged but, instead, are attributable to artifacts introduced bythe acquisition and processing of the image. Sources of noise include,for example, quantum exposure noise, dark current noise, thermal noise,readout noise, and others. Since image noise is inversely proportionalto the size of the imaging device, the problem of noise is especiallyacute for smaller imaging devices. When image acquisition is performedaboard mobile platforms such as unmanned aerial vehicles (UAVs), theimage noise problem is especially acute both because of the smallercameras used on the mobile platforms and because of noise introduced bymovement of the mobile platforms. In view of the foregoing, there is aneed for improved color correction systems and methods that increasecolor correction accuracy while limiting noise amplification.

The present disclosure sets forth systems and methods for colorcorrection of a digital image which overcome shortcomings of existingcolor correction techniques by increasing color correction accuracywhile limiting noise amplification. Based on color reference images,color correction parameters are calibrated to increase color correctionaccuracy while limiting noise amplification. The calibration can beperformed in the CIE L*a*b* color space to more closely reflect humanperception of distances between colors. The calibration can be performedwith reference to a virtual noisy image that can be sampled at differentspatial frequencies. At each spatial frequency, a peak signal-to-noiseratio (PSNR) can be used to evaluate the amount of noise introduced bycolor correction. The color correction parameters can be optimized byusing a genetic process. A two-step parameter optimization method can beused that avoid the optimization process being trapped in local optima.The present systems and methods advantageously are suitable for use, forexample, by unmanned aerial vehicles (UAVs) and other mobile platforms.

Turning now to FIG. 1, an exemplary color correction apparatus 100 isshown as including a processor 110 and a memory 120. The processor 110can comprise any type of processing system. Exemplary processors 110 caninclude, without limitation, one or more general purpose microprocessors(for example, single or multi-core processors), application-specificintegrated circuits, application-specific instruction-set processors,graphics processing units, physics processing units, digital signalprocessing units, coprocessors, network processing units, audioprocessing units, encryption processing units, and the like. In certainembodiments, the processor 110 can include an image processing engine ormedia processing unit, which can include specialized hardware forenhancing the speed and efficiency of certain operations for imagecapture, image filtering, and image processing. Such operations include,for example, Bayer transformations, demosaicing operations, whitebalancing operations, color correction operations, noise reductionoperations, and/or image sharpening/softening operations. In certainembodiments, the processor 110 can include specialized hardware and/orsoftware for performing various color correction parameter calibrationfunctions and operations described herein. Specialized hardware caninclude, but are not limited to, specialized parallel processors,caches, high speed buses, and the like.

The memory 120 can comprise any type of memory and can be, for example,a random access memory (RAM), a static RAM, a dynamic RAM, a read-onlymemory (ROM), a programmable ROM, an erasable programmable ROM, anelectrically erasable programmable ROM, a flash memory, a secure digital(SD) card, and the like. Preferably, the memory 120 has a storagecapacity that accommodates the needs of the color correction parametercalibration functions and operations described herein. The memory 120can have any commercially-available memory capacity suitable for use inimage processing applications and preferably has a storage capacity ofat least 512 Megabytes, 1 Gigabyte, 2 Gigabytes, 4 Gigabytes, 16Gigabytes, 32 Gigabytes, 64 Gigabytes, or more. In some embodiments, thememory 120 can be a non-transitory storage medium that can storeinstructions for performing any of the processes described herein.

The color correction apparatus 100 can further include any hardwareand/or software desired for performing the color correction parametercalibration functions and operations described herein. For example, thecolor correction apparatus 100 can include one or more input/outputinterfaces (not shown). Exemplary interfaces include, but are notlimited to, universal serial bus (USB), digital visual interface (DVI),display port, serial ATA (SATA), IEEE 1394 interface (also known asFireWire), serial, video graphics array (VGA), super video graphicsarray (SVGA), small computer system interface (SCSI), high-definitionmultimedia interface (HDMI), audio ports, and/or proprietaryinput/output interfaces. As another example, the color correctionapparatus 100 can include one or more input/output devices (not shown),for example, buttons, a keyboard, keypad, trackball, displays, and/or amonitor. As yet another example, the color correction apparatus 100 caninclude hardware for communication between components of the colorcorrection apparatus 100 (for example, between the processor 110 and thememory 120).

Turning now to FIG. 2, an exemplary embodiment of an imaging system 200is shown as including a color correction apparatus 100, an image sensor130, and a color filter 140. The color correction apparatus 100 can beprovided in the manner discussed in more detail above with reference toFIG. 1. The memory 120 of the color correction apparatus 100 is shown asstoring color correction parameters 125, noise generation parameters126, pre-correction and post-correction image data 127, and intermediatevalues 128 produced during various color correction parametercalibration functions and operations described herein. The image sensor130 can perform the function of sensing light and converting the sensedlight into electrical signals that can be rendered as an image. Variousimage sensors 130 are suitable for use with the disclosed systems andmethods, including, but not limited to, image sensors 130 used incommercially-available cameras and camcorders. Suitable image sensors130 can include analog image sensors (for example, video camera tubes)and/or digital image sensors (for example, charge-coupled device (CCD),complementary metal-oxide-semiconductor (CMOS), N-typemetal-oxide-semiconductor (NMOS) image sensors, and hybrids/variantsthereof). Digital image sensors can include, for example, atwo-dimensional array of photosensor elements that can each capture onepixel of image information. The resolution of the image sensor 130 canbe determined by the number of photosensor elements. The image sensor130 can support any commercially-available image resolution andpreferably has a resolution of at least 0.1 Megapixels, 0.5 Megapixels,1 Megapixel, 2 Megapixels, 5 Megapixels, 10 Megapixels, or an evengreater number of pixels. The image sensor 130 can have specialtyfunctions for use in various applications such as thermography, creationof multi-spectral images, infrared detection, gamma detection, x-raydetection, and the like. The image sensor 130 can include, for example,an electro-optical sensor, a thermal/infrared sensor, a color ormonochrome sensor, a multi-spectral imaging sensor, a spectrophotometer,a spectrometer, a thermometer, and/or an illuminometer.

The color filter 140 is shown in FIG. 2 as separating and/or filteringincoming light based on color and directing the light onto theappropriate photosensor elements of the image sensor 130. For example,the color filter 140 can include a color filter array that passes red,green, or blue light to selected pixel sensors to form a color mosaic(not shown). The layout of different colors on the color mosaic can bearranged in any convenient manner, including a Bayer pattern. Once acolor mosaic is formed, a color value of each pixel can be interpolatedusing any of various demosaicing methods that interpolate missing colorvalues at each pixel using color values of adjacent pixels. As analternative to filtering and demosaicing, the image sensor 130 caninclude an array of layered pixel photosensor elements that separateslight of different wavelengths based on the properties of thephotosensor elements. In either case, an image can be acquired by theimage sensor 130 as intensity values in each of a plurality of colorchannels at each pixel.

The imaging system 200 is further shown in FIG. 2 as acquiring an imageof a color reference 150 to perform calibration of color correctionparameters 125. The color reference 150 preferably has a known referencecolor value C_(ref) that is known or that can be otherwise determined inadvance, making the color reference 150 suitable for use as a colorstandard. Stated somewhat differently, the reference color value C_(ref)is a property of the color reference 150 that is independent of how thecolor reference 150 is imaged. The reference color value C_(ref) can bedesignated based on an average human perception of the color reference150. The reference color value C_(ref) can thus serve as an objectivemeasure how a color imaged by the image sensor 130 can be corrected soas to match the average human perception.

The color reference 150 is preferably, but not necessarily, homogeneousin color. Flatness of the color reference 150 is preferable, though notessential, to avoid variations attributable to differential lightscattering. The optical properties of the color reference 150 need notbe ideal for purposes of performing color correction, so long as theoptical properties do not interfere with imaging the color reference150. The color reference 150 can be made of one or more of a variety ofmaterials such as plastic, paper, metal, wood, foam, composites thereof,and other materials. Furthermore, the color, reflectance, and/or otheroptical properties of the color reference 150 can advantageously becalibrated as desired using an appropriate paint or other coating. Insome embodiments, the color reference 150 can advantageously includemultiple color patches 151, each of which has a different referencecolor value C_(ref). This embodiment enables multiple color references150 to be imaged at the same time, reducing the number of image captureoperations for color correction. This embodiment is particularlysuitable when a large number of color references 150 are to be imaged inorder to calibrate color correction parameters 125 with greateraccuracy. Commercially available color references 150 include, forexample, MacBeth ColorChecker, MacBeth ColorChecker SG, and the like.

Although images acquired by the image sensor 130 are described above inan RGB (red, green, and blue) color space for illustrative purposesonly, the images can be acquired in other color spaces, as well. Thecolor space in which images are acquired depends generally on theproperties of the image sensor 130 and any color filters 140.Furthermore, the color space in which an image is acquired need not bethree-dimensional but can have any number of dimensions as desired tocapture the spectral composition of the image. The number of dimensionscan depend on the number of color channels of the image sensor 130. Thecolor space of an acquired image can be one-dimensional,two-dimensional, three-dimensional, four-dimensional, five-dimensional,or more.

Once acquired by the image sensor 130, an image can be converted betweencolor spaces as desired for processing and/or calibration. For example,a conversion from a sRGB color space with coordinates (R_(sRGB),G_(sRGB), B_(sRGB)) to a CIE 1931 XYZ color space with coordinates (X,Y, Z) entails a linear conversion, which can be represented by thefollowing three-dimensional matrix:

$\begin{matrix}{\begin{bmatrix}X \\Y \\Z\end{bmatrix} = {\begin{bmatrix}0.4124 & 0.3576 & 0.1805 \\0.2126 & 0.7152 & 0.0722 \\0.0193 & 0.1192 & 0.9505\end{bmatrix}\begin{bmatrix}R_{sRGB} \\G_{sRGB} \\B_{sRGB}\end{bmatrix}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

In some embodiments, it can be desirable to express the color of animage in a non-linear color space. One suitable non-linear color spacefor imaging applications is a CIE L*a*b* color space (for example, a CIE1976 L*a*b* color space) as defined by the International Commission onIllumination. The color of an image in a CIE L*a*b* color space can becomputed from the colors of the image in a CIE 1931 XYZ color spaceusing the following non-linear transformation:

$\begin{matrix}{L^{*} = {{116{f\left( \frac{Y}{Y_{n}} \right)}} - 16}} & {{Equation}\mspace{14mu} (2)} \\{a^{*} = {500\left\lbrack {{f\left( \frac{X}{X_{n}} \right)} - {f\left( \frac{Y}{Y_{n}} \right)}} \right\rbrack}} & {{Equation}\mspace{14mu} (3)} \\{b^{*} = {200\left\lbrack {{f\left( \frac{Y}{Y_{n}} \right)} - {f\left( \frac{Z}{Z_{n}} \right)}} \right\rbrack}} & {{Equation}\mspace{14mu} (4)} \\{{{where}\mspace{14mu} {f(t)}} = \left\{ \begin{matrix}t^{1/3} & {{{if}\mspace{14mu} t} > \left( \frac{6}{29} \right)^{3}} \\{{\frac{1}{3}\left( \frac{29}{6} \right)^{2}t} + \frac{4}{29}} & {otherwise}\end{matrix} \right.} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

In the above equations (2)-(5), X_(n), Y_(n), and Z_(n) are the CIE XYZvalues of the color at a reference white point. The CIE L*a*b* colorspace is designed to mimic the color response of human perception. Thenon-linearity of the transformation from the CIE XYZ color space to theCIE L*a*b* color space reflects the nonlinearity of human perception.Representing a color in the CIE L*a*b* color space has the advantagethat the CIE L*a*b* color space is perceptually uniform to human beings,meaning that a change of a given amount in a color value will produce aproportional change of visual significance. According, calibration ofcolor correction parameters 125 can advantageously be performed afterconverting input and reference colors into a CIE L*a*b* color spacerepresentation.

In some embodiments, it can be desirable to express the color of animage in a YUV color space (for example, a Y′UV color space). The YUVcolor space is represented by one luminance component Y representingimage brightness and two chrominance components U and V representingimage color. A conversion from a RGB color space with coordinates (R, G,B) to a YUV color space with coordinates (Y, U, V) entails a linearconversion, which can be represented by the following three-dimensionalmatrix:

$\begin{matrix}{\begin{bmatrix}Y \\U \\V\end{bmatrix} = {\begin{bmatrix}0.299 & 0.587 & 0.114 \\{- 0.14713} & {- 0.28886} & 0.436 \\0.615 & {- 0.51499} & {- 0.10001}\end{bmatrix}\begin{bmatrix}G \\B \\R\end{bmatrix}}} & {{Equation}\mspace{14mu} (6)}\end{matrix}$

Although conversions between specific color spaces are shown anddescribed for illustrative purposes only, an image can be converted fromany first predetermined color space to any other second predeterminedcolor space as desired.

Turning now to FIG. 3, an alternative embodiment of the imaging system200 is shown. The imaging system 200 of FIG. 3 includes a colorcorrection apparatus 100, which is shown as obtaining several inputs forcalibration of color correction parameters 125. Without limitation, animage sensor 130 is shown as acquiring an image of a color reference150. The image is then passed to the color correction apparatus 100,which can obtain an input color value C_(in) of the image. The inputcolor value C_(in) represents a pre-color-corrected value that reflectsthe image acquisition properties of the image sensor 130, filteringproperties of an image filter 140, as well as any other opticalproperties of the imaging system 200. In one embodiment, the input colorvalue C_(in) can be transformed from the color space of the colorreference image to a non-linear color space—for example, a CIE L*a*b*color space. The transformation can be performed, for example, by firstusing a linear transformation from a RGB color space to an intermediateCIE XYZ color using Equation (1) shown above. The color values in theintermediate CIE XYZ color space can be non-linearly transformed to aCIE L*a*b* color space as shown above in Equations (2)-(5). Suchtransformations can be performed on a processor 110 (shown in FIG. 1) ofthe color correction apparatus 100.

Similarly, the color correction apparatus 100 can obtain a referencecolor value C_(ref) that corresponds to the input color value C_(in) forcolor reference 150. If desired, the reference color value C_(ref) canbe transformed into a non-linear color space—for example, the CIE L*a*b*color space. In some embodiments, the reference color value C_(ref)advantageously can be directly inputted into the color correctionapparatus 100 in the CIE L*a*b* color space, thereby making thetransformation step unnecessary.

In FIG. 3, the color correction apparatus 100 is further shown asobtaining noise evaluation color values C_(noise) from a noiseevaluation image 160. The noise evaluation image 160 can be any imagecontaining noise. As color correction tends to amplify noise, the noiseevaluation image 160 can be used to calibrate color correctionparameters 125 in order to limit noise amplification. Stated somewhatdifferently, the noise evaluation image 160 can used to evaluate nownoise is amplified with a given set of color correction parameters 125(shown in FIG. 2), and thereby select a set of color correctionparameters 125 with reduced noise amplification. In one embodiment, thenoise evaluation color values C_(noise) can be transformed into the YUVcolor space, as further described below with reference to FIG. 7. Thetransformation can be performed, for example, using the lineartransformation from the RGB color space to the YUV color space shownabove in Equation (6). This transformation can be performed using theprocessor 110 of the color correction apparatus 100.

In one embodiment, the noise evaluation image 160 can be an imageacquired by the image sensor 130 with or without filtering through thecolor filter 140. In this embodiment, the noise evaluation image 160 ispreferably an image of the color reference 150. Imaging the colorreference 150 advantageous allows the simultaneous determination of theinput color values C_(in) and the noise evaluation color valuesC_(noise).

Alternatively and/or additionally, the noise evaluation image 160 can bea virtual noise evaluation image 160A. The virtual noise evaluationimage 160A can be generated by the color correction apparatus 100 usinga pre-determined set of noise generation parameters 126 (shown in FIG.2). The noise generation parameters 126 can, for example, reflect thedistribution of the noise that is generated virtually (for example,Poisson or Gaussian noise). The specific noise generation parameters 126can reflect the types of noise that the imaging system 200 can beexpected to encounter in usage. A virtual noise evaluation image 160Acan be used because the evaluation of noise amplification does notrequire information about the color of an underlying object that isimaged. Instead, an arbitrary image containing noise can be evaluatedfor how the noise of that image would be amplified under a given set ofcolor correction parameters 125. For example, the noise evaluation colorvalues C_(noise) of the virtual noise evaluation image 160A can berepresented as follows:

C _(noise) =C _(noise) _(_) _(free) ±n  Equation (7)

where C_(noise) _(_) _(free) represents the color of the virtual noiseevaluation image 160A before noise is added, and n represents the noiseadded.

Once the inputs for color correction parameter calibration (for example,input color values C_(in), reference color values C_(ref), and noiseevaluation color values C_(noise)) are obtained by the color correctionapparatus 100, these inputs can be stored for later use by the colorcorrection apparatus 100 (for example, in a memory 120 as shown in FIG.1). For example, the inputs for color correction parameter calibrationcan be obtained as part of an initialization process for a new imagingdevice 200 prior to usage. The inputs for color correction parametercalibration can be stored in the memory 120 and called upon periodicallyto re-calibrate the color correction parameters 125 as desired (forexample, as image response characteristics of the imaging device 200change after wear and tear). The inputs for color correction parametercalibration can be, but do not need to be, re-obtained for each newcolor correction parameter calibration.

Turning now to FIG. 4, an exemplary top-level method 400 of calibratingcolor correction parameters 125 is shown. The method 400 advantageouslycan be applied to calibrating the color correction parameters 125 for adigital imaging device 200 (shown in FIGS. 2 and 3). At 401, input colorvalues C_(in) and reference color values C_(ref) are obtained for eachof a plurality of color references 150 (shown in FIGS. 2 and 3).Preferably, the input color values C_(in) and reference color valuesC_(ref) are obtained or transformed into a non-linear color space—forexample, a CIE L*a*b* color space—as described above with reference toFIG. 3. Additionally, a noise evaluation image 160 having a color noisefor evaluating noise reduction is obtained.

At 402, a plurality of color correction parameters 125 are adjusted soas to optimize a fitness function J. In some embodiments, the fitnessfunction J can comprise a color correction error e_(color) and/or anoise amplification metric D_(noise) based on the input color valuesC_(in), the reference color values C_(ref), and the noise evaluationimage 160. An exemplary embodiment of the adjusting is described in moredetail below with respect to FIG. 5.

Turning now to FIG. 5, an exemplary method 500 of calibrating colorcorrection parameters 125 (shown in FIG. 2) for a digital imaging device200 (shown in FIGS. 2 and 3) is shown. At 501, input color (orpre-correction) values C_(in) for a color references 150 are colorcorrected using the current values of the color correction parameters125 to obtain post-correction input color values Ĉ_(in). This operationcan be represented as:

Ĉ _(in) =CC(C _(in))  Equation (8)

In the above equation (8), CC represents a color correction operation.The specific implementation of the color correction operation CC dependson the underlying form of the color correction parameters 125. In oneembodiment, the color correction parameters 125 can take the form of amatrix having dimensions n×m, where m is dimensionality of thepre-correction color value and n is the dimensionality of thepost-correction color value. In this embodiment, the color correctionoperation CC will take the form of a matrix multiplication thattransforms an m-dimensional color value vector into an n-dimensionalcolor value vector. Preferably, the pre-correction color value and thepost-correction color value have the same dimensionality, in which caseCC will take the form of a square matrix. Preferably, the pre-correctioncolor value and the post-correction color value are eachthree-dimensional (for example, for color values in the RGB, CIE XYZ,CIE L*a*b*, and LUV color spaces), in which case CC will take the formof a 3×3 matrix. An advantage of using a matrix is that a matrix candescribe a color correction operation CC using only n×m correctionparameters 125, allowing decreased memory usage. However, linear colorcorrection using a matrix may be unsuitable for some applications.

In another embodiment, the color correction parameters 125 can take theform of a look-up table (LUT) indexed in m dimensions that containsordered m-tuples (a₁, a₂, . . . , a_(m)) each mapping to ann-dimensional vector, where m is dimensionality of the pre-correctioncolor value and n is the dimensionality of the post-correction colorvalue. Preferably, the look-up table is three-dimensional, that is,indexed in three dimensions. An advantage of using a look-up table toimplement the color correction parameters 125 is that a look-up tablecan account for a non-linear relationship between a pre-correction colorvalue and a post-correction color value. Furthermore, since the entriesin the look-up table are discrete, interpolation operations can beperformed when pre-correction color values fall in between discreteentries. Such interpolation operations can include finding look-up tableentries that have the closest distance (for example, Euclidian distance)to the pre-correction color value, and interpolating a corrected colorvalue using the closest look-up table entries. For example, linearinterpolations can be performed for one-dimensional look-up tables, andmulti-linear interpolations can be performed for look-up tables inhigher dimensions. In this embodiment, the color correction operation CCwill take the form of a look-up operation in the look-up table, followedby an interpolation operation, if desired. The color correctionparameters 125 can be implemented in multiple ways simultaneously; forexample, a combination of a matrix and a look-up table can be used.

In one embodiment, a Shepard interpolation can be used to perform colorcorrection where the color correction parameters 125 take the form of alook-up table (LUT). In one embodiment, a color-corrected value for agiven color p can be found as follows:

$\begin{matrix}{{f(p)} = \frac{\sum\limits_{i = 1}^{n}{{\hat{f}\left( c_{i} \right)}{w_{i}\left( {{p - c_{i}}} \right)}}}{\sum\limits_{i = 1}^{n}{w_{i}\left( {{p - c_{i}}} \right)}}} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

In the above equation (9), i is an index over the different input colorvalues C_(in) and their corresponding reference color values C_(ref),c_(i) represents the ith value of the input color values C_(in),{circumflex over (f)}(c_(i)) represents the ith value of the referencecolor values C_(ref), (∥p−c_(i)∥) represents a distance (for example, aEuclidian distance) between the given color p and c_(i), and w_(i)represents a weight of the ith input color value C_(in).

At 502, the post-correction input color values Ĉ_(in) are compared withthe reference color values C_(ref), and the color correction errore_(color) is computed based on the comparison. For example, where thepost-correction input color values Ĉ_(in) and reference color valuesC_(ref) are represented in a CIE L*a*b* color space, the colorcorrection error e_(color) can be expressed as:

$\begin{matrix}{e_{color} = \sqrt{\sum\limits_{j}^{j \in {\{{L^{*}a^{*}b^{*}}\}}}\left( {C_{{i\; n\; \_ \; j}\;} - {\hat{C}}_{{i\; n\; \_ \; j}\;}} \right)^{2}}} & {{Equation}\mspace{14mu} (10)}\end{matrix}$

In the above equation (10), C_(in) _(_) _(j) and Ĉ_(in) _(_) _(j)represent the jth component of the reference color values C_(ref) andthe post-correction input color values Ĉ_(in), respectively. Statedsomewhat differently, the color correction error e_(color) is theEuclidian distance between the post-correction input color values Ĉ_(in)and the reference color values Cretin the color space in which the colorvalues are represented. Where the color correction error e_(color) is tobe determined over multiple color references 150 (or, equivalently, overmultiple color patches 151 of a given color reference 150), the colorcorrection error e_(color) can be taken as a weighted and/or unweightedaverage over the color patches 151.

At 503, noise evaluation color values C_(noise) are color correctedusing the current values of the color correction parameters 125 toobtain post-correction noise evaluation color values Ĉ_(noise). Thisoperation can be represented as:

Ĉ _(noise) =CC(C _(noise))  Equation (11)

In the above equation (11), CC represents a color correction operationas described above with reference to 501. The specific color correctionoperation CC depends on the implementation of the color correctionparameters 125 and, as described above with reference to 501, can takethe form of a matrix or a look-up table with each form having respectiveadvantages.

At 504, the post-correction noise evaluation color values Ĉ_(noise) arecompared with pre-correction noise evaluation color values C_(noise),and the noise amplification metric D_(noise) is found based on thecomparison. The noise amplification metric D_(noise) can be any measureof the distance between post-correction noise evaluation color valuesĈ_(noise) and the pre-correction noise evaluation color valuesC_(noise). That is, the greater the value of the noise amplificationmetric D_(noise), the more noise is amplified after applying a colorcorrection.

Where the noise amplification metric D_(noise) is to be determined overmultiple color references 150 (or, equivalently, over multiple colorpatches 151 of a given color reference 150), the noise amplificationmetric D_(noise) can be taken as a weighted and/or unweighted averageover the color patches 151. In one embodiment, the noise amplificationmetric D_(noise) can be taken as a weighted average over the colorpatches 151.

$\begin{matrix}{D_{noise} = \frac{\sum\limits_{i = 1}^{N}{\omega_{i}D_{i}}}{\sum\limits_{i = 1}^{N}\omega_{i}}} & {{Equation}\mspace{14mu} (12)}\end{matrix}$

In the above equation (12), i is an index over the color patches 151, Nis the total number of color patches 151, and ω_(i) is a non-negativeweight for color patch i. The weights ω_(i) can be set according to thesensitivity of the average human perception to the color of each colorpatch 151. For example, colors having greater sensitivity of humanperception can be given greater weights ω_(i).

At 505, a fitness function J can be determined. In some embodiments, thefitness function J can be found as a weighted and/or unweighted sum ofthe color correction error e_(color) and the noise amplification metricD_(noise). For example, an unweighted fitness function J can berepresented as the following sum:

J=e _(color) +D _(noise)  Equation (13)

In some embodiments, a weighted fitness function J can used toadvantageously weight the color correction error e_(color) more than thenoise amplification metric D_(noise), or vice versa. The amount ofweighting for the fitness function J can be determined, for example, byrepeating a color correction parameter calibrations for differentweights and taking the weight that gives the best (for example, thelowest) value of the fitness function J. Alternatively and/oradditionally, the amount of weighting for the fitness function J can bedetermined based on prior color correction parameter calibrations (forexample, using different imaging devices).

Turning now to FIG. 6, an exemplary method 600 for calibrating colorcorrection parameters 125 (shown in FIG. 2) is shown as including twosteps. At 601, a first optimization process is applied to obtain initialvalues CC₀ for the color correction parameters 125. The firstoptimization preferably samples broadly the space of possible colorcorrection parameter values so as to avoid becoming trapped in localoptima. Any of various optimization processes can be used in the firstoptimization at 601, including a genetic process, a simulated annealingmethod, and other non-greedy methods that avoid local optima. At 602, asecond optimization process is applied using the initial values CC₀ as astarting point to obtain further optimized values CC_(opt) for the colorcorrection parameters 125. At the second optimization, at 602, a goal isto find the local optimum value. Accordingly, direct optimizationmethods are suitable for the second optimization at 602. Exemplarydirect optimization methods include, but are not limited to, gradientdescent methods.

Turning now to FIG. 7, an exemplary genetic process 700 is shown forcalibrating color correction parameters 125 (shown in FIG. 2). A geneticprocess is an optimization method loosely based on evolutionaryprinciples in biology, where possible solutions to a problem aregenerated as members of a “population,” and the members are selectedbased on a fitness function over a number of selection rounds. Thegenetic process 700 can be used to find an optimal solution to theproblem of selecting a set of color correction parameters 125 tooptimize (for example, minimize) the fitness function J that includes acolor correction error e_(color) and a noise amplification metricD_(noise). At 701, a predetermined number N of initial sets of candidatecolor correction parameters 125A are selected as the initial“population” of solutions. The predetermined number N can comprise anysuitable number of initial sets and, for example, can be at least 10,50, 100, 500, 1000, or more. The initial population of the N sets ofcandidate color correction parameters 125A can be selected, for example,by sampling the space of possible parameters at specified intervals.Alternatively and/or additionally, the sampling can be done at random.

At 702, the fitness function J is evaluated for the members of the“population,” that is, for each of the N sets of candidate colorcorrection parameters 125A. From among the N initial sets of candidatecolor correction parameters 125A, the initial set that has the bestvalue of the fitness function J (for example, the minimal value, if thefitness function J to be minimized) is chosen. At 703, if the best valuepasses a predefined threshold, the genetic process stops at 704.Alternatively and/or additionally, at 705, if certain conditions are met(for example, the genetic process has been run for more than a certainnumber of rounds, or the genetic process has not produced more than aspecific amount of improvement in the fitness function J from the priorround), the genetic process stops at 704. After the genetic processstops, at 704, the candidate color correction parameters 125A giving thebest value of the fitness function J is declared to be the “winner,” andthese candidate color correction parameters 125A can be outputted and/orused as a starting point for further optimization.

If the “best” candidate correction parameters 125 do not pass apredefined threshold and/or certain conditions for stopping the geneticprocess, at 703, are not met, the genetic process continues, at 706, bydiscarding and replacing candidate color correction parameters 125Ahaving the lowest values of the fitness function J. In one embodiment, agiven percentile of the candidate color correction parameters 125Ahaving the lowest fitness function J can be discarded and replaced withnew candidate color correction parameters 125A. The new candidate colorcorrection parameters 125A can, for example, be generated in the sameway as the initial candidate color correction parameters 125A. In someembodiments, at least 10%, 20%, 30%, 40%, 50%, or more the lowestscoring fitness functions J can be discarded.

At 707, “mutation” operations can be applied to the candidate colorcorrection parameters 125A, simulating biological mutations ofchromosomes between successive generations of individuals. Here, eachset of candidate color correction parameters 125A can be conceptuallytreated as a “chromosome” that is also subject to mutation. Mutations tothe candidate color correction parameters 125A include, for example,“point mutations” changing individual parameters at random and/or“crossover” mutations between two sets of candidate color correctionparameters 125A. For example, where the candidate color correctionparameters 125A take the form of a matrix, a crossover can be performedby swapping corresponding rows and/or columns or portions thereofbetween two candidate matrices. Where the candidate color correctionparameters 125A take the form of a look-up table, a crossover can beperformed by swapping one or more corresponding entries in the look-uptable. Once the mutations are applied to the candidate color correctionparameters 125A, the method 700 can return to 702 for evaluating thefitness function J for the new round of the genetic process.

The noise amplification metric D_(noise) can be determined using anysuitable approach, including but not limited to using peaksignal-to-noise ratios (PSNR) and/or color variances.

Turning now to FIG. 8, an exemplary diagram is shown for finding thenoise amplification metric D_(noise) using a peak signal-to-noise ratios(PSNR). Beginning with noise-free color values C_(noise) _(_) _(free),noise-evaluation color values C_(noise) can be found by adding noise n,as described in Equation (7). A color correction CC can be applied tothe noise-free color values C_(noise) _(_) _(free) and noise-evaluationcolor values C_(noise), respectively, to find correspondingpost-correction values of the noise-free color values {umlaut over(C)}_(noise) _(_) _(free) and noise evaluation color values Ĉ_(noise).For example, the color correction is shown in Equation (11) and inEquation (14) below:

C _(noise) _(_) _(free) =CC(C _(nosie) _(_) _(free))  Equation (14)

Based on the parameters C_(noise) _(_) _(free), C_(noise), Ĉ_(noise)_(_) _(free), and Ĉ_(noise), a pair of PSNR values PSNR and

can be found through determining a mean squared error (MSE), as shown inEquations (15) through (18):

$\begin{matrix}{{MSE} = \frac{{{c_{noise} - c_{{noise}\; \_ \; {free}}}}_{2}^{2}}{\Sigma_{j}s_{j}}} & {{Equation}\mspace{14mu} (15)} \\{{PSNR} = {10{\log_{10}\left( \frac{{MAX}_{I}^{2}}{MSE} \right)}}} & {{Equation}\mspace{14mu} (16)}\end{matrix}$

In the above equations (15)-(16), S, i.e. Σ_(j)S_(j), is the number ofpixels and MAX_(I) is the maximum value of C_(noise) and C_(noise) _(_)_(free), and j is an index over virtual color patches.

=  c ^ noise - c ^ noise   _   free  2 2 Σ j  s j Equation   (17 ) = 10  log 10  ( MAX I 2 ) Equation   ( 18 )

In the above equations (17)-(18), S, i.e. Σ_(j)S_(j), is the number ofpixels and MAX_(I) is the maximum value of Ĉ_(noise) and Ĉ_(noise) _(_)_(free), and/is an index over virtual color patches

In some embodiments, determining the noise amplification metricD_(noise) can include finding a PSNR difference that is a differencebetween a PSNR for the pre-correction noise evaluation image and a

for the corrected noise evaluation image.

In some embodiments, the noise amplification metric D_(noise) can bedetermined by downsampling. For example, the downsampling can be aspatial downsampling, as illustrated in FIG. 9. In particular, FIG. 9illustrates an embodiment of downsampling in which an image (forexample, images having pre-correction noise evaluation color valuesC_(noise), or pre-correction noise-free color values C_(noise) _(_)_(free)) is sample at every other pixel in a first downsampling. In someembodiments, the downsampled image can be downsampling again, and thedownsampling process can be repeated as often as desired up to Miterations. Although not shown in FIG. 9, a similar downsampling processcan be performed for images that have been color-corrected (for example,images having post-correction noise evaluation color values Ĉ_(noise) orpost-correction noise-free color values Ĉ_(noise) _(_) _(free)). Sincedownsampling can be an iterative process, color values and PSNR valuesat particular iterations are denoted with a subscript from 0 to Mcorresponding to the iteration, as shown in FIGS. 8-9.

After each round of downsampling, the downsampled images can be used todetermine one or more downsampled PSNRs as well as a downsampled PSNRdifference. Returning to FIG. 8, pre-correction noise-free color valuesC_(noise) _(_) _(free) ₁ and pre-correction noise evaluation colorvalues C_(noise) ₁ that have undergone one round of downsampling can beused to find a corresponding downsampled PSNR₁. Likewise,post-correction noise-free color values Ĉ_(noise) _(_) _(free) ₁ andpost-correction noise evaluation color values Ĉ_(noise) ₁ that haveundergone one round of downsampling can be used to find a correspondingdownsampled

_(i). After M downsampling rounds, a set of PSNR values PSNR_(i) and

_(i) will be obtained, where i ranges from 0 to M. The set of PSNRvalues can be used to find corresponding PSNR differences for value of iranging from 0 to M, where i=0 corresponds to a PSNR difference that hasnot been downsampled, and i=m corresponds to a PSNR difference that hasbeen downsampled m times.

In some embodiments, the noise amplification metric D_(noise) can beobtained by taking a weighted average of a PSNR difference and at leastone downsampled PSNR difference. In some embodiments, the noiseamplification metric D_(noise) can be obtained by taking a weightedaverage of the PSNR difference and the plurality of successivelydownsampled PSNR differences. The weight applied to each PSNR differenceand/or downsampled PSNR difference can represented as w_(i), where iranges from 0 to M. An exemplary method of finding the noiseamplification metric D_(noise) is shown as follows in Equation (19),which is reproduced in FIG. 8:

$\begin{matrix}{D_{noise} = \frac{\sum\limits_{i = 0}^{M}{w_{i}\left( {{PSNR}_{i} -} \right)}}{\sum\limits_{i = 0}^{M}w_{i}}} & {{Equation}\mspace{14mu} (19)}\end{matrix}$

where M is the total number of downsampling iterations and w_(i) is theweight given to each downsampling iteration i.

In some embodiments, at least one of the weights w_(i) is non-zero.Stated somewhat differently, PSNR differences at one or more iterationsi can be given a weight of zero to effectively ignore that PSNRdifference, provided that not all of the weights are ignored.

Turning now to FIG. 10, an exemplary method 1000 is shown for findingthe noise amplification metric D_(noise) that locates and compares peaksignal-to-noise ratios (PSNR) at successively downsampled frequencies.At 1001 a, an initial value of a pre-correction PSNR PSNR₀ can be foundusing the pre-correction noise evaluation color values C_(noise) ₀ andpre-correction noise-free color values C_(noise) _(_) _(free) ₀ , asdescribed above with reference to FIGS. 8 and 9. At 1002 a, C_(noise) ₀and C_(noise) _(_) _(free) ₀ can each be downsampled to obtain C_(noise)₁ and C_(noise) _(_) _(free) ₁ , respectively. At 1003 a, a downsampledPSNR_(i) can be found from C_(noisei) and C_(noise) _(_) _(free) ₁ .Optionally, at 1004 a, the process of downsampling and finding acorresponding downsampled PSNR can be repeated for M iterations, asdesired.

Similarly, the iterative downsampling process can be repeated forcolor-corrected images. At 1001 b, an initial value of a post-correctionPSNR

₀ can be found using the post-correction noise evaluation color valuesĈ_(noise) ₀ and post-correction noise-free color values Ĉ_(noise) _(_)_(free) ₀ , as described above with reference to FIGS. 8 and 9. At 1002b, Ĉ_(noise) ₀ and Ĉ_(noise) _(_) _(free) ₀ can each be downsampled toobtain Ĉ_(noise) ₁ and Ĉ_(noise) _(_) _(free) ₁ , respectively. At 1003b, a downsampled

₁ can be found from Ĉ_(noise) ₁ and Ĉ_(noise) _(_) _(free) ₁ .Optionally, at 1004 b, the process of downsampling and finding acorresponding downsampled

can be repeated for M iterations, as desired.

Finally, at 1005, the set of PSNR values and color-corrected PSNR valuesfound at iterations 0 to M can be used to find the noise amplificationmetric D_(noise)—for example, as shown above in Equation (19).

In another embodiment, the noise amplification metric D_(noise) can beobtained based on a variance of Y, U, and V components of thepre-correction noise evaluation color values C_(noise) ₀ andpost-correction noise evaluation color values Ĉ_(noise) ₀ . In anexemplary embodiment, the noise amplification metric D_(noise) can beobtained using the Equation (20):

$\begin{matrix}{D_{noisy} = \frac{\sum\limits_{i = 0}^{M}{w_{i}\begin{pmatrix}{\frac{{Var}_{{noisy}_{Y_{i}}}}{{Var}_{{noise}\; \_ \; {input}_{Y_{i}}}} +} \\{\frac{{Var}_{{noisy}_{U_{i}}}}{{Var}_{{noise}\; \_ \; {input}_{U_{i}}}} + \frac{{Var}_{{noisy}_{V_{i}}}}{{Var}_{{noise}\; \_ \; {input}_{U_{i}}}}}\end{pmatrix}}}{\sum\limits_{i = 0}^{M}w_{i}}} & {{Equation}\mspace{14mu} (20)}\end{matrix}$

wherein

Var_(noise_input_(Y_(i)))

represents the variance of the Y components of the pre-correction noiseevaluation color values C_(noise) ₁ , and

C_(noise_(i)), and  Var_(noisy_(Y_(i)))

represents the variance of the Y components of the post-correction noiseevaluation color values Ĉ_(noise) _(i) , and likewise for the U and Vcomponents, and w_(i) is the weight given to each downsampling iterationi, where w_(i)≧0.

Turning now to FIG. 11, an exemplary embodiment of the imaging system200 is shown wherein the imaging system 200 is shown as being installedaboard an unmanned aerial vehicle (UAV) 1100. A UAV 1100, colloquiallyreferred to as a “drone,” is an aircraft without an onboard human pilotand whose flight is controlled autonomously and/or by a remote pilot.The imaging system 200 is suitable for installation aboard any ofvarious types of UAVs 1100, including, but not limited to, rotocraft,fixed-wing aircraft, and hybrids thereof. Suitable rotocraft include,for example, single rotor, dual rotor, trirotor, quadrotor (quadcopter),hexarotor, and octorotor rotocraft. The imaging system 200 can beinstalled on various portions of the UAV 1100. For example, the imagingsystem 200 can be installed within a fuselage 1110 of the UAV 1100.Alternatively, the imaging system 200 can be mounted onto an exteriorsurface 1020 (for example, on the underside 1025) of the UAV 1100.Furthermore, the various components of the imaging system 200 can beinstalled on the same portion, and/or different portions, of the UAV1100. For example, an image sensor 130 can be mounted on an exteriorsurface 1120 to facilitate image acquisition; while, a color correctionapparatus 100 advantageously can be installed within the fuselage 1110for protection against wear and tear. Likewise, the various componentsof the color correction apparatus 100 can be installed on the sameportion, and/or different portions, of the UAV 1100. Although shown anddescribed with respect to a UAV 1100 for purposes of illustration only,the imaging system 200 can include, or be mounted on, any type of mobileplatform. Exemplary suitable mobile platforms include, but are notlimited to, bicycles, automobiles, trucks, ships, boats, trains,helicopters, aircraft, various hybrids thereof, and the like.

Example 1

The following color correction parameter calibration experiment wasperformed to determine the efficacy of a method of calibration withnoise regulation in comparison to the method without noise regulation.First, an input image was used to calibrate color correction parametersby using a fitness function that includes the noise amplification metricin the manner described above. FIG. 12 shows a chrominance diagram ofresulting color errors in a CIE L*a*b* color space (showing a crosssection in the a* and b* dimensions), showing a mean color correctionerror of 16.8 with a maximum color correction error of 29.7. FIG. 13shows a plot of the resulting noise levels from the same experiment,showing that the average Y (luminance) noise is 0.83%; while, theaverage chrominance noise in the R, G, and B components are 1.38%,1.31%, and 1.63%, respectively.

In contrast, the same input image was used to calibrate color correctionparameters by using a fitness function that does not include the noiseamplification metric. FIG. 14 shows a chrominance diagram of resultingcolor errors in a CIE L*a*b* color space, showing a mean colorcorrection error of 17.7 with a maximum color correction error of 35,both of which are significantly greater than the corresponding errorsobtained with noise regulation. FIG. 15 shows a plot of thecorresponding noise levels of the experiment, showing that the average Y(luminance) noise is 0.86%; while, the average chrominance noise in theR, G, and B components are 1.56%, 1.31%, and 1.66%, respectively, whichare significantly greater than the noise obtained by calibration withnoise regulation. Accordingly, it can be seen from this experiment thatcolor correction parameter calibration with noise regulation is animprovement over color correction parameter calibration without noiseregulation.

Example 2

The following color correction parameter calibration experiment wasperformed to determine the efficacy of a method of calibration withconversion to a CIE L*a*b* color space in comparison to the method thatperforms the calibration in a CIE XYZ color space. First, an input imagewas used to calibrate color correction parameters after the input andreference colors of the input image are converted to a CIE L*a*b*.Optimization yielded the following matrix of color correctionparameters:

$M_{1} = {\quad\left\lbrack {\begin{matrix}0.47009134530394125 & 0.30369624777814247 & 0.226212406917916 \\0.1126102415 & 0.5888365492340442 & 0.29855320919654277 \\0.07360346735208151 & {- 0.258973359} & 1.1853698917599211\end{matrix} {\quad \left. \quad \right\rbrack}} \right.}$

having an optimized e_(c) of 2.412169304.

Next, the same input image was used to calibrate color correctionparameters after the input and reference colors of the input image areconverted to a CIE XYZ. Optimization yielded the following matrix ofcolor correction parameters:

$M_{2} = {\quad\begin{bmatrix}1.11570755070485 & 0.160204597011795 & 0.044497 \\{- 0.473878528884461} & 1.60653254387421 & {- 0.132654014989749} \\{- 0.0543893885667206} & {- 0.8115292} & 1.86591858820061\end{bmatrix}}$

having an optimized e_(c) of 3.0107447. This comparison shows that usinga non-linear color space—here, a CIE L*a*b* color space—yields improvedresults over using a CIE XYZ color space.

Example 3

The following example shows the process of optimizing a set of colorcorrection parameters using the two-step method of FIG. 6. In the firststep of the two-step method, a genetic process is used to find a set ofinitial parameters so as to avoid becoming trapped in local optima. Thefitness value of the parameters for the genetic process over six hundredgenerations is shown in FIG. 16 at the upper panel, showing that thefitness value reaches a best value of 335.134 after 600 generations. Inthe second step of the two-step method, a direct optimization process isused starting from the initial parameters produced at the end of stepone. In the second step, after another 600 generations, the directoptimization method reduces the average distance between the correctedinput colors and the corresponding reference colors, as shown in FIG. 15at the lower panel. This example shows that it is advantageous to use atwo-step optimization method.

The disclosed embodiments are susceptible to various modifications andalternative forms, and specific examples thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the disclosed embodiments are not to belimited to the particular forms or methods disclosed, but to thecontrary, the disclosed embodiments are to cover all modifications,equivalents, and alternatives.

What is claimed is:
 1. A method for calibrating a digital imaging devicefor color correction, comprising: obtaining input color values andreference color values for respective color references, the input colorvalues and reference color values being in a non-linear color space;obtaining a noise evaluation image having a color noise for evaluatingnoise reduction; and adjusting a plurality of color correctionparameters based on the input color values, reference color values, andthe noise evaluation image using a fitness function.
 2. The method ofclaim 1, wherein said obtaining the input color values comprises imagingthe color references each having a plurality of color patches.
 3. Themethod of claim 1, wherein said obtaining the noise evaluation imagecomprises obtaining a virtual noise evaluation image.
 4. The method ofclaim 1, wherein said adjusting comprises adjusting the color correctionparameters using the fitness function having a color correction errorand a noise amplification metric.
 5. The method of claim 4, wherein saidadjusting comprises determining the color correction error by: colorcorrecting the input color values; and comparing the corrected inputcolor values with the reference color values.
 6. The method of claim 4,wherein said adjusting comprises determining the noise amplificationmetric by: color correcting the noise evaluation image using the colorcorrection parameters; and comparing the corrected noise evaluationimage with the pre-correction noise evaluation image.
 7. The method ofclaim 6, wherein said determining the noise amplification metriccomprises determining the noise amplification metric using a peaksignal-to-noise ratio (PSNR).
 8. The method of claim 7, wherein saiddetermining the noise amplification metric comprises finding a PSNRdifference that is a difference between a PSNR for the pre-correctionnoise evaluation image and a PSNR for the corrected noise evaluationimage.
 9. The method of claim 8, wherein said determining the noiseamplification metric comprises determining a downsampled PSNR differenceby: downsampling the pre-correction noise evaluation image to obtain adownsampled pre-correction noise evaluation image; downsampling thecorrected noise evaluation image to obtain a downsampled corrected noiseevaluation image; and finding the downsampled PSNR difference as adifference between a PSNR for the downsampled pre-correction noiseevaluation image and a PSNR for the downsampled corrected noiseevaluation image.
 10. The method of claim 6, wherein said determiningthe noise amplification metric comprises determining the noiseamplification metric by converting the color corrected noise evaluationimage into a YUV color space.
 11. The method of claim 4, furthercomprising determining the noise amplification metric as a weighted sumof noise amplification levels of each of a plurality of color patches ofthe noise evaluation image.
 12. The method of claim 1, wherein saidadjusting the color correction parameters comprises performing look-upoperations and interpolation operations in a look-up table.
 13. Themethod of claim 12, wherein said adjusting the color correctionparameters comprises performing an interpolation operation that is aShepard interpolation.
 14. An apparatus, comprising: a processorconfigured to: receive input color values and reference color values forrespective color references, the input color values and reference colorvalues being in a non-linear color space; receive a noise evaluationimage having a color noise for evaluating noise reduction; and adjust aplurality of color correction parameters based on the input colorvalues, reference color values, and the noise evaluation image using afitness function.
 15. The apparatus of claim 14, wherein the fitnessfunction comprises a color correction error and a noise amplificationmetric.
 16. The apparatus of claim 15, wherein said processor isconfigured to determine the color correction error by: color correctingthe input color values; and comparing the corrected input color valueswith the reference color values.
 17. The apparatus of claim 15, whereinsaid processor is configured to determine the noise amplification metricby: color correcting the noise evaluation image using the colorcorrection parameters; and comparing the corrected noise evaluationimage with the pre-correction noise evaluation image.
 18. The apparatusof claim 17, wherein said processor is configured to determine the noiseamplification metric using a peak signal-to-noise ratio (PSNR).
 19. Theapparatus of claim 18, wherein said processor is configured to determinethe noise amplification metric by finding a PSNR difference that is adifference between a PSNR for the pre-correction noise evaluation imageand a PSNR for the corrected noise evaluation image.
 20. The apparatusof claim 19, wherein said processor is configured to determine the noiseamplification metric by determining a downsampled PSNR difference by:downsampling the pre-correction noise evaluation image to obtain adownsampled pre-correction noise evaluation image; downsampling thecorrected noise evaluation image to obtain a downsampled corrected noiseevaluation image; and finding the downsampled PSNR difference as adifference between a PSNR for the downsampled pre-correction noiseevaluation image and a PSNR for the downsampled corrected noiseevaluation image.
 21. The apparatus of claim 15, wherein the noiseevaluation image comprises a plurality of color patches, and wherein thenoise amplification metric is determined as a weighted sum of noiseamplification levels of each of the color patches.
 22. The apparatus ofclaim 14, wherein the apparatus is mounted aboard an unmanned aerialvehicle (UAV).
 23. An imaging device, comprising: an image sensor forimaging a plurality of color references; and a processor configured to:receive input color values and reference color values for respectivecolor references, the input color values and reference color valuesbeing in a non-linear color space; receive a noise evaluation imagehaving a color noise for evaluating noise reduction; and adjust aplurality of color correction parameters based on the input colorvalues, reference color values, and the noise evaluation image using afitness function.
 24. The imaging device of claim 23, wherein theimaging device is mounted aboard an unmanned aerial vehicle (UAV).
 25. Anon-transitory readable medium with instructions stored thereon that,when executed by a processor, perform the steps comprising: obtaininginput color values and reference color values for respective colorreferences, the input color values and reference color values being in anon-linear color space; obtaining a noise evaluation image having acolor noise for evaluating noise reduction; and adjusting a plurality ofcolor correction parameters based on the input color values, referencecolor values, and the noise evaluation image using a fitness function.26. The non-transitory readable medium of claim 25, wherein saidadjusting comprises adjusting the color correction parameters using thefitness function having a color correction error and a noiseamplification metric.
 27. The non-transitory readable medium of claim26, wherein said adjusting comprises determining the color correctionerror by: color correcting the input color values; and comparing thecorrected input color values with the reference color values.
 28. Thenon-transitory readable medium of claim 26, wherein said adjustingcomprises determining the noise amplification metric by: colorcorrecting the noise evaluation image using the color correctionparameters; and comparing the corrected noise evaluation image with thepre-correction noise evaluation image.
 29. The non-transitory readablemedium of claim 28, wherein said determining the noise amplificationmetric comprises determining the noise amplification metric using a peaksignal-to-noise ratio (PSNR).
 30. The non-transitory readable medium ofclaim 29, wherein said determining the noise amplification metriccomprises finding a PSNR difference that is a difference between a PSNRfor the pre-correction noise evaluation image and a PSNR for thecorrected noise evaluation image.